The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads, is a commercially significant application of polymer film, including generically, polyethylene. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods. Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The roll of film so used may be installed on a hand-held wrapping tool for ease of use by the operator.
Some of the properties desired of a good stretch wrap film are as follows: good cling or cohesion properties, good transparency, low stress relaxation with time, high resistance to transverse tear when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, high modulus of elasticity, high puncture resistance.
Physical properties which are particularly significant for the successful use of thermoplastic films in stretch wrap applications include their puncture resistance, their elongation characteristics, their toughness, and their resistance to tearing while under tension. In general, tensile toughness is measured as an area under a stress-strain curve developed for a thermoplastic film, and it may be considered as the tensile energy absorbed, expressed in units of ft.-lbs./cu.in. to elongate a film to break under tensile load. In turn, this toughness characteristic is a function of the capacity of such films to elongate. The process of stretching the film decreases that capacity. Accordingly, the stretch wrap process will decrease the toughness of the film while it is in its stretched condition as an overwrap as compared to unstretched counterparts, including such materials as shrink wrap. Generally this loss of toughness is proportional to the amount of stretch imparted to the film as it is overwrapping a load of goods.
As hereinabove indicated, the resistance to tear characteristic of such films will obviously be an important physical characteristic for stretch wrap applications since if the edge of the stretch film roll is nicked, abraded or in any way weakened before stretching or during the stretching operation, premature tearing of the film will usually occur during wrapping or subsequent handling of the load of goods.
Some resins which have been used in the fabrication of stretch wrap film are polyethylene, polyvinylchloride and ethylene vinyl acetate. A fairly recent development has been the utilization of linear low density polyethylene (LLDPE) in the manufacture of stretch wrap film, e.g., as described in U.S. Pat. Nos. 4,399,180, 4,418,114 and 4,518,654, the contents of which are incorporated by reference herein. The excellent toughness and puncture resistance properties of LLDPE makes it an excellent resin for such an application. LLDPE and methods for its manufacture are described in, among others, U.S. Pat. Nos. 3,645,992, 4,076,698, 4,011,382, 4,163,831, 4,205,021, 4,302,565, 4,302,566, 4,359,561 and 4,522,987. In general, films fabricated from the typical LLDPE resins of commerce possess little or no cling property on either surface thereof in the absence of added cling agent. LLDPE films possessing an inherent cling property wherein the LLDPE possesses a relatively high level of n-hexane extractables are disclosed in application Ser. No. 039,892, filed Apr. 17, 1987, the contents of which are incorporated by reference in their entirety.
Other thermoplastic films possessing a cling property are known in the art. U.S. Pat. No. 4,311,808 describes a cling film containing a homogeneous mixture of polyisobutylene, ethylene-propylene and a low density polyethylene.
U.S. Pat. No. 4,367,256 describes a cling wrap plastic film based on a blend of high pressure low density polyethylene (HPLDPE) and LLDPE in which the latter resin represents from 5-16 weight percent of the total. In one embodiment, this film is sandwiched between two HPLDPE films.
U.S. Pat. No. 4,399,173 describes a multilayer plastic film free of melt fracture which is suitable for a variety of applications including, by implication, the stretch wrapping of goods. The film possesses a core layer of LLDPE resin of melt index 0.2-3.0 decigrams per minute and skin layers of LLDPE resin of melt index 5.0-20.2 decigrams per minute.
U.S. Pat. Nos. 4,399,180 and 4,418,114 describe a coextruded composite stretch wrap film in which an LLDPE core layer is surfaced with HPLDPE skin layers.
In the one-sided cling stretch wrap film of U.S. Pat. No. 4,518,654, layer A of the disclosed A/B structure is fabricated from a resin possessing an inherent cling property and/or a cling property resulting from the incorporation of a cling additive therein. Layer A is coextruded with layer B, which is fabricated from a resin exhibiting little if any cling. Layer B further exhibits a slide property when in contact with a layer of itself with relative motion therebetween. Layer B can contain an anti-cling additive at a level of from about 0.05 to about 2.0 weight percent of the resin component of the layer, and can consist of such materials as crystalline and amorphous silicate. In each of the two working examples of this patent, layer A is an LLDPE film containing a cling additive, namely, polyisobutylene, with synthetic sodium silicate particles being uniformly incorporated throughout layer B of the film of Example 1 and amorphous silica particles being uniformly incorporated throughout layer B of the film of Example 2. Layer B, the layer providing the noncling surface of the films of Examples 1 and 2 is HPLDPE. Improved one-sided cling/one-sided slip stretch wrap films are disclosed in application Ser. No. 249,525, filed Sep. 26, 1988.
Other multi-layer composite films known include those disclosed in U.S. Pat. No. 4,364,981 in which polyethylene films comprising a core or intermediate layer of low pressure low density polyethylene (LPLDPE) and skin layers of high pressure low density polyethylene (HPLDPE) are taught. The LPLDPE described within U.S. Pat. No. 4,364,981 is actually an LLDPE, as may be seen by reference to the process for making LLDPE of U.S. Pat. No. 4,011,382 disclosed at column 1, lines 52-55. The films disclosed are said to be useful in forming bags such as trash bags. No disclosure of possible utility as a stretch wrap may be found within this patent.
U.S. Pat. No. 4,565,720 discloses three layer packaging bags in which an intermediate layer comprising a mixture of LLDPE and high density polyethylene (HDPE) is taught. Outer layers of the bags are made of HPLDPE or a mixture of HPLDPE and an ethylene-vinyl acetate copolymer. The structures disclosed are not cited as being useful in forming stretch wrap films.
A multi-layer film in which the intermediate layer comprises LLDPE and HDPE and the outer layers comprise an LLDPE and at least one such outer layer further comprises an HPLDPE is disclosed in U.S. Pat. No. 4,574,104. One film disclosed therein is a three-layer structure in which the intermediate layer comprises LLDPE, HPLDPE and HDPE. Packaging bags are the chief utility cited for these multi-layer films. No disclosure of utility as a stretch wrap is made.
U.S. Pat. No. 4,511,609 discloses a multi-layer film for use in the manufacture of garbage bags wherein the film comprises a first outer layer of LLDPE, a second outer layer of HPLDPE and a intermediate layer also of HPLDPE or a blend of LLDPE and HPLDPE. No statement as to whether the films disclosed have any possible utility as stretch wraps is made within this patent.
In U.S. Pat. No. 4,657,811, a three-layer plastic film suitable for use in trash bag manufacturing is disclosed wherein an intermediate layer of foamed HPLDPE is disposed between outer layers of LLDPE. Again, the films disclosed are not cited as finding utility as stretch wrap films.
As previously indicated, methods of stretch wrapping articles, containers, etc., are known. U.S. Pat. No. 3,986,611 describes a tension-wrapped palletized load obtained with a stretch wrap film possessing a cling additive.
U.S. Pat. No. 4,079,565 describes a stretch-wrapped package, process and apparatus which employs a stretch wrap polyethylene film.
U.S. Pat. No. 4,409,776 discloses a method and apparatus for packaging articles with a composite stretch wrap film, one surface of which is nonadhesive. The adhesive surface is obtained with an "adhesive film" such as one fabricated from ethylene-vinyl acetate copolymer, 1,2-polybutadiene or styrenebutadiene copolymer and the nonadhesive surface is obtained with a "nonadhesive film" such as one fabricated from a crystalline polyolefin, e.g., polyethylene, polypropylene or ethylene-propylene copolymer.
While it is apparent that a wide variety of stretch wrap films have been disclosed for use in packaging or palletizing operations, most suffer from one or more notable deficiencies. Not all films known in the art possess good on-load stretched cling properties. Other films are deficient in their tensile properties, including the force required in the machine direction (MD) to stretch the film. Also, not all end use applications require the same film characteristics, necessitating the complex production of a broad range of films to suit these applications.
Therefore, what is needed is a stretch wrap film having improved strength and toughness, with superior film cling and reduced unwind friction characteristics, which is suitable for all load types encountered in pallet wrapping applications.